Recently, utilization of coal as an energy source in place of petroleum has become more prevalent. In order to widely utilize coal which is inferior in its physical and chemical properties as a fuel to those of petroleum, development of processing and distribution of coal and of technology for promoting the utilization of coal has been in urgent demand. Research and development of a pulverized coal incinerating boiler and the fluidized bed boiler in the field of combustion technology have been positively advanced. With respect to combustion technology such as the above, utilization is restricted to certain kinds of coals in view of combustion efficiency, requirements of low NOx and low SOx. Also, problems such as the complexity of coal feeding systems and difficulty in controlling load fluctuations have become evident, which problems have been particularly evidenced in small and medium size boilers.
Fluidized bed boilers can be classified into two types as noted below according to the difference in a system wherein arrangement of heat transfer portions and combustion of unburnt particles flowing out from the fluidized bed are taken into account.
(1) Non-recycling type fluidized bed boilers (which are referred to as conventional type fluidized bed boilers or bubbling type fluidized bed boilers) PA1 (2) Recycling type fluidized bed boilers
In a non-recycling type, a heat transfer tube is arranged within a fluidized bed, and heat exchange is carried out by physical contact between the burning fuel and a fluidizing medium with high heat transfer efficiency. On the other hand, in a recycling type, fine unburnt materials, ash and/or a part of the fluidizing medium (recycling solid) are merged into a flow of combustion gas and guided to a heat exchanging portion arranged independently of the combustion chamber where combustion of the unburnt particles is continued and the circulating solid having undergone heat exchange is returned to the combustion chamber, the aforesaid title being given since the solid is recycled.
A non-recycling and a recycling type fluidized bed boiler will be described with reference to FIGS. 4 and 5.
FIG. 4 shows a non-recycling type fluidized bed boiler, in which air for fluidization fed under pressure from a blower (not shown) is injected from an air chamber 74 into a boiler 71 through a diffusion plate 72 to form a fluidized bed 73, and fuel, for example, granular coal, is supplied to the fluidized bed 73 for combustion. Heat transfer tubes 76 and 77 are provided in the fluidized bed 73 and an exhaust gas outlet of a free board portion, respectively, to recover thermal energy.
Exhaust gas cooled to a relatively low temperature is guided from an exhaust gas outlet of the free board portion to a convection heat transfer portion 78 to recover thermal energy and is discharged outside the system after contained particles are recovered at a cyclone 79. Ash recovered in the convection heat transfer portion is taken out through a tube 81 and discharged outside the system via a tube 82 together with ash taken out from a tube 80, a part thereof being returned to the fluidized bed 73 for reburning through the air chamber 74 or a fuel inlet 75.
FIG. 5 shows a recycling type fluidized bed boiler, in which air for fluidization fed under pressure from a blower (not shown) is blown from an air chamber 104 into a furnace 101 through a diffusion plate 102 to fluidize and burn granular coal containing lime as a desulfurizing agent to be supplied into the furnace as needed.
Unlike a non-recycling type fluidized bed boiler, injecting speed of fluidizing air blown through the diffusion plate 102 is higher than the terminal speed of the fluidizing particles, and therefore mixing of particles and gas is more actively effected and the particles are blown upward together with gas so that a fluidizing layer and a jet-stream layer are formed in that order from the bottom over the whole zone of the combustion furnace. The particles and gas are guided to a cyclone 108 after a small amount of heat exchange is effected at a water cooling furnace wall 107 provided along the flow path. The combustion gas passed through the cyclone 108 undergoes heat exchange at a convection heat transfer portion 109 arranged in a flue at the rear portion.
On the other hand, the particles collected at the cyclone 108 are again returned to the combustion chamber via a flow passage 113, and a part of the particles is guided to an external heat exchanger 115 via a passage 114 for the purpose of controlling the furnace temperature, and after being cooled it is again returned to the combustion chamber, although part thereof may be discharged outside the system as ash. A feature lies in that the particles are recycled into the combustion chamber in a manner as just described. The recycling particles are mainly limestone supplied as a desulfurizing agent, burnt ash of supplied coal and unburnt ash, etc.
In these fluidized bed boilers, a wide variety of materials can be burnt in view of characteristics of the combustion system thereof, but some disadvantages thereof have been noted.
The disadvantages of the bubbling type fluidized bed boiler are problems such as those regarding load characteristics, complexity of the fuel supply system and abrasion of heat transfer tubes in the bed, etc.
In order to solve the problems inherent in such matters as those described above, a recycling type apparatus has become desirable. However, some further factors need to be developed in order to maintain the temperature of a recycling system including a cyclone of a combustion furnace at a proper value. In addition, there still remains a problem in the handling of the recycling solid. With respect to small and medium type boilers, it is difficult to make them compact.